Slot antenna

ABSTRACT

An antenna in a radar level gauge for determining the level of the surface of a medium stored in a tank. The antenna has a first and a second electrically conductive layer, and between the conductive layers a dielectric layer. The second of the conductive layers is provided with a number of holes in a predetermined pattern. Leading to the center of the dielectric layer is a transmission line. The microwaves from a radar unit in the radar level gauge are supplied to the dielectric layer in the antenna via the transmission line and transmitted out in a radial direction from a feed point in the dielectric layer that acts as a waveguide. Via said holes in the second conductive layer a microwave beam radiates out. Due to matching of the phase of the microwave and the placing of the individual holes in the pattern that the holes form, a predominant waveguide mode of the desired order is obtained.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Section 371 National Stage Application ofInternational Application No. PCT/SE02/02081, filed Nov. 14, 2002 andpublished as WO 03/042639 on May 22, 2003, in English.

FIELD OF THE INVENTION

The present invention relates to an antenna for use in a level gaugethat utilizes a radar for determining the level of the surface of amedium stored in a tank, the invention presenting both a device and amethod for causing a predominant and predetermined microwave mode to betransmitted from the antenna.

BACKGROUND OF THE INVENTION

It is known to measure the level of the surface of a medium stored in atank by using radar level gauges. Examples of such a device are given inU.S. Pat. No. 4,641,139. In a device of the type described in saidpatent specification, a tube that communicates with the medium in thetank is used as a waveguide for the microwaves that are transmitted by aradar unit and are received by the same radar unit following reflectionfrom the surface of the medium in the tube.

Tubes that are used as waveguides in level measurement are usuallycircular and have a diameter that is at least twice as great as thewavelength of the microwaves that are used in the radar unit. Whenmicrowaves are transmitted in a tube with the given conditions, aplurality of different modes of microwaves is transmitted in the tubewaveguide. To obtain the only desired microwave mode, a mode generatorthat comprises a waveguide is used in this case for generating amicrowave mode of a higher order and in addition a long, cone-shapedantenna as a transition stage from the narrower waveguide to the widertube in the tank for transmitting the desired mode in the tubewaveguide.

The present invention presents an alternative design of an antenna, inwhich the design of the antenna renders a separate mode generatorsuperfluous.

A known antenna structure for achieving a waveguide mode by means of aflat antenna is presented in patent specification EP 1 083 413.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an antenna is presented for aradar level gauge for determining the level of the surface of a mediumthat is stored in a tank. A tube with a diameter that is at least twiceas great as the wavelength of the microwaves that are transmitted isused as a waveguide Inside the tank. The antenna has a first and asecond electrically conductive layer, and between the conductive layersa dielectric layer. The second of the conductive layers is provided witha number of holes in a predetermined pattern. A transmission line leadsto the dielectric layer. The microwaves from a radar unit in the radarlevel gauge are supplied to the dielectric layer in the antenna via thetransmission line and transmitted out in a radial direction from a feedpoint in the dielectric layer that acts as a waveguide. Via said holesin the second conductive layer, a microwave beam radiates out into thespace that surrounds said second conductive layer. Due to matchingbetween the phase of the microwave and the placing of the individualholes in a pattern formed by the holes, a predominant waveguide mode ofthe desired order is obtained.

According to a second aspect of the invention, a method is presented forachieving a desired and pure waveguide mode according to the independentmethod claim.

In the measurements carried out by means of the device and methodrespectively according to the invention, the desired predominant mode isan H₀₁ mode.

The feeding of a microwave signal to the dielectric layer preferablytakes place at the centre of this layer, when the layers in the antennaare circular in form. The microwave beam radiated is fixed preferably toradiate out orthogonally to the layers of the antenna. By adjusting thepattern of holes, other angles between the beam radiated and the antennalayers can be achieved.

One advantage of an antenna according to the aspects of the invention isthat the device Is simpler than according to the previously known methodfor producing the desired waveguide mode. Another advantage is that amode generator intended for the purpose for achieving a predominant modeis not required.

In the known device according to EP 1 083 413, the antenna function hasmore parts than in the invention presented, including more dielectriclayers. In the device according to the invention, no feed network isused to which the microwave signal is supplied, such as is the case withthe antenna according to said patent. One dielectric layer is usedinstead as a supply medium for the microwave signal, where thedielectric layer feeds the signal radially outwards from the centre ofthe dielectric layer, the actual holes or slots in the second conductivelayer acting as antenna elements. Furthermore, the antenna can be mademore robust, since the conductive planes, in the form of the conductivelayers, are not so demanding with regard to tolerances. They can be cutout of metal layers or etched out of printed circuit board laminate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows diagrammatically a radar level gauge in a tank fordetermining the level of the surface of a medium stored in a tank.

FIG. 2 shows diagrammatically the different layers that together buildup the antenna according to the aspects of the invention.

FIG. 3 shows an example of a slot pattern for achieving a predeterminedmicrowave mode from the antenna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of an embodiment is described here with reference to enclosedfigures.

The principle of a radar level gauge is shown in FIG. 1. A tank 1 isused for storage of a medium 2. The medium can be a liquid, such as oil,refined products and gas in liquid form, or can consist of a particulatematerial, i.e. a powdered solid substance. A radar 3 is fitted to thetop 4 of the tank 1, from which a microwave beam is transmitted from andreceived by the radar via an antenna 5 on the inside of the tank. Thebeam transmitted is reflected from the surface 6 of the medium andpicked up by the antenna 5. Through comparison and evaluation in acalculation and control unit of the time delay when using microwavesbetween a transmitted and reflected signal characteristic, the level ofthe surface 6 of the medium is determined in a known manner. The figurealso shows that the microwaves are transmitted via a tube-shapedwaveguide 7 that communicates with the medium, which in this case is aliquid, through openings (not shown) in the wall of the waveguide.

The construction of an antenna 10 according to the invention is shown inFIG. 2. The antenna is intended to transmit and receive a microwavesignal in a radar level gauge according to the above. In the figure, afirst electrically conductive layer 11 is illustrated. Thus the firstconductive layer 11 can be formed of a metal layer, for example a thincopper plate. A dielectric layer 12 is applied next to the firstconductive layer 11 and adjoining this. The dielectric layer has athickness that is determined by the frequency and propagation velocityof the microwave signal. The thickness shall thus be less than half thewavelength for the selected frequency. The material in the dielectriclayer can be teflon, but can also consist of any dielectric that hassufficiently low losses and withstands the environment in the spacewhere the antenna is placed, such as e.g. the temperature in the space.The next layer in the antenna 10 consists of a second conductiveelectrical layer 13, which adjoins the other side of the dielectriclayer 12, i.e. the side of said dielectric layer that faces away fromthe first electrically conductive layer 11. This second electricallyconductive layer is also best executed as a metal layer of copper oranother conductive substance. A number of holes has been punched or cutout in this second electrically conductive layer 13. In the exampleshown, these holes consist of oblong slots 14 that are executed in acertain pattern. In the example shown, all layers are shown as circularlayers, since the antenna in the example is used for transmitting andreceiving microwaves in a circular waveguide. The appearance of thepattern is determined by the field image that it is intended to achieve.The theory for calculating antenna parameters, such as the position ofthe slots and the slot geometry in the desired microwave mode isindicated in the article: IEEE Trans. on Antennas and Propagation, Vol.AP-45, No. 7, July 1997, P. W. Davis & M. E. Bialkowski. The antennasdescribed therein are intended for a normal antenna function, i.e.generally parabolic-type antennas. In the present case, generation of apredominant waveguide mode is already taken into account in the antenna,the pattern being calculated for this purpose.

The first 11 and second conductive layer 13 are advantageously earthedto prevent electrostatic charging of the layers. Earthing can beachieved via connection of the first layer to an earthed envelope of thetransmission line. Earthing of the second layer is effected for exampleby a conductive connection at the periphery of the layers between thefirst and second conductive layer. An alternative is for both conductivelayers 11, 13 to be connected to surrounding conductive material, suchas e.g. the tube waveguide 7 in which the antenna 10 is fastened.

The antenna Is supplied with a microwave signal from the radar unit ofthe radar level gauge 3 via a transmission line 15. The first conductivelayer 11 is provided with a lead-through 16 in the form of an openingmade in the layer. The signal-carrying part of the transmission line 15is designed to be led in via the lead-through 16 and terminated in acavity that is formed centrally in the dielectric layer 12. The cavitycan be a recess that matches in size and receives e.g. thesignal-carrying third wire in a coaxial cable that serves as atransmission line. Due to this arrangement and the fact that thedielectric layer is located between two electrically conductive layers,the microwave signal will be propagated radially outwards from thecavity 17 to the periphery of the dielectric layer 12 like a wavemotion, in which the dielectric layer acts as a waveguide. An absorbentlayer can be applied at the periphery of the dielectric layer to obtainbetter performance. The absorbent layer is formed here as a ring or edgethat encloses the dielectric layer 12 along its periphery and is locatedbetween the two electrically conductive layers 11, 13.

As already said, the second electrically conductive layer 13 is providedwith a number of slots 14. The wave motion in the form of the microwavesignal that is propagated in the dielectric layer 12 will leak outthrough the holes that the slots form in the second conductive layer 13,in which case the microwave energy will radiate out from said secondconductive layer 13, the device acting like an antenna. By adapting thepositions of the slots 14 to the microwave signal used, the desiredmicrowave mode can be made to be generated. This occurs when thefrequency of the microwave signal and its phase interact constructivelywith the position of the slots 14 in the second conductive layer 13. Dueto constructive interference between the wave parts leaking fromrespective slots, these are assembled to form a desired microwave mode.The theoretical calculations behind this represent the prior art and arenot described here. The microwave mode that is desirable here is theH₀₁, mode (alternative description TE₀₁), since this mode is well suitedto level measurement in said tube that is used as a waveguide in thetank 1. Said mode is suitable for use in the type of measurementenvisaged, since it gives low attenuation in the intended tube waveguide7 even when the tube waveguide is rusty or fouled. Other modes of themicrowave signal only occur to a low degree, attenuation of such modesnot needing to be undertaken when using the antenna structure described.The slot pattern that is shown in FIG. 2 is a specific pattern linked toa certain waveguide mode, in the example shown for generating the H₀₁mode.

If the antenna 10 is placed across the tube waveguide 7, i.e. at rightangles to the tube, the microwave beam transmitted is disposed to beorthogonal to the antenna surface, i.e. to the plane of the conductivelayers. If on the other hand it is desired that the antenna 10 issloping in relation to the horizontal plane, the pattern of slots isexecuted in such a way that the microwave energy radiated from therespective slot through constructive interference creates a microwavebeam that is angled diagonally to the antenna surface.

1. Antenna for a radar in a level gauge that is used to determine thelevel of the surface of a medium in a tank, characterized in that itcomprises: a first and a second electrically conductive layer, adielectric layer between the conductive layers, a number of holesexecuted in a predetermined pattern in the second conductive layer and atransmission line set up to the dielectric layer via a connection,wherein microwaves from the radar are supplied to the dielectric layervia the transmission line's connection and are distributed out radiallyin the dielectric layer from the connection and from the dielectriclayer radiate out with a predetermined predominant microwave mode viathe holes in the second conductive layer; and wherein the respectiveholes in the pattern and the geometry of the holes are based onpredetermined microwave mode as well as the microwave's frequency andphase position at respective holes.
 2. Antenna according to claim 1, inwhich the microwaves are fed to the dielectric layer via a connectionarranged centrally in the layer.
 3. Antenna according to claim 1, inwhich the first layer is executed with a lead-through for thetransmission line for supplying a microwave signal to the dielectriclayer.
 4. Antenna according to claim 1, in which a cavity is formed inthe dielectric layer for receiving the feeding part of the transmissionline for transmitting microwaves out into the surrounding dielectriclayer.
 5. Antenna according to claim 1 in which the holes have beenplaced in such a pattern that the microwave mode transmitted will be anH₀₁ Mode.
 6. Antenna according to claim 5, in which the patterncomprises a number of holes executed as slots, which are arranged inzigzag lines of slots, wherein the zigzag lines lie along radii thatstart out from a point by the feed point of the transmission line. 7.Antenna according to claim 1, in which at least one of the conductivelayers is earthed.
 8. Antenna according to claim 1, in which the antennatransmits and receives a microwave signal reflected by the surface via atube, which is used as a waveguide for the microwaves that aretransmitted and received respectively by the radar.
 9. Antenna accordingto claim 1, in which the antenna transmits and receives a microwavesignal reflected by the surface via a tube, which is used as a waveguidefor the microwaves that are transmitted and received respectively by theradar and in which the tube has a diameter that is at least twice asgreat as the wavelength of the microwaves.
 10. Antenna according toclaim 1, in which the thickness of the dielectric layer is less thanhalf the wavelength of the microwave that is transmitted via theantenna.
 11. Method for generating a predominant microwave mode fortransmission of said mode from an antenna in a level gauge, in which aradar is used to determine the level of the surface of a medium in atank, in which the method comprises the stages: a microwave signal istransmitted from the radar via a transmission line to a dielectriclayer, a first and a second electrically conductive layer are arrangedon each side of and adjacent the dielectric layer, the microwave signalis transmitted radially outwards from the transmission line in thedielectric layer that serves as a waveguide, in the second of theelectrically conductive layers the microwave signal is permitted to passa number of holes that form a predetermined pattern and by adapting thehole pattern to the frequency and phase of the microwave signal, thepredominant microwave mode is obtained, which radiates out via the holesin the second conductive layer.
 12. Antenna for a radar in a level gaugethat is used to determine the level of the surface of a medium in atank, characterized in that it comprises: a first and a secondelectrically conductive layer, a dielectric layer between the conductivelayers, a number of holes executed in a predetermined pattern in thesecond conductive layer; a transmission line set up to the dielectriclayer via a connection, wherein microwaves from the radar are suppliedto the dielectric layer via the transmission line's connection and aredistributed out radially in the dielectric layer from the connection andfrom the dielectric layer radiate out with a predetermined predominantmicrowave mode via the holes in the second conductive layer; wherein therespective holes in the pattern and the geometry of the holes are basedon predetermined microwave mode as well as the microwave's frequency andphase position at respective holes; and wherein the antenna transmitsand receives a microwave signal reflected by the surface via a tube,which is used as a waveguide for the microwaves that are transmitted andreceived respectively by the radar.
 13. Antenna according to claim 12,in which the positions of the respective holes in the pattern and thegeometry of the holes are based on predetermined microwave mode as wellas the microwave's frequency and phase position at respective holes. 14.Antenna according to claim 12, in which the microwaves are fed to thedielectric layer via a connection arranged centrally in the layer. 15.Antenna according to claim 12, in which the first layer is executed witha lead-through for the transmission line for supplying a microwavesignal to the dielectric layer.
 16. Antenna according to claim 12, inwhich a cavity is formed in the dielectric layer for receiving thefeeding part of the transmission line for transmitting microwaves outinto the surrounding dielectric layer.
 17. Antenna according to claim12, in which the holes have been placed in such a pattern that themicrowave mode transmitted will be an H₀₁ Mode.
 18. Antenna according toclaim 17, in which the pattern comprises a number of holes executed asslots, which are arranged in zigzag lines of slots, wherein the zigzaglines lie along radii that start out from a point by the feed point ofthe transmission line.
 19. Antenna according to claim 12, in which atleast one of the conductive layers is earthed.
 20. Antenna according toclaim 12, in which the antenna transmits and receives a microwave signalreflected by the surface via a tube, which is used as a waveguide forthe microwaves that are transmitted and received respectively by theradar and in which the tube has a diameter that is at least twice asgreat as the wavelength of the microwaves.
 21. Antenna according toclaim 12, in which the thickness of the dielectric layer is less thanhalf the wavelength of the microwave that is transmitted via theantenna.